Factor XIII, the last enzyme activated in the coagulation cascade, stabilizes blood clots by producing covalent crosslinks between fibrin subunits. It's a member of the transglutaminase class of enzymes which catalyze the formation of covalent bonds between glutamine and lysine residues in a number of biological materials including blood clots and extracellular matrix. To better understand the structural aspects of this process, the three-dimensional molecular structure of factor XIII and its activated and inhibited forms will be obtained using X-ray crystallographic techniques. These studies will determine which amino acids in the protein are involved in the enzyme mechanism and whether the enzyme possesses unique structural elements for dealing with large macromolecular substrates such as fibrin or fibronectin. The clinical symptoms of factor XIII deficiency (hemorrhagic episodes involving rebleeding after initial clot formation) can be alleviated by therapeutic doses of the enzyme. The enzyme is also one component in fibrin glue, a material useful in surgical applications, and is being investigated for its involvement in atherosclerosis. The enzyme is important for linking molecules together in larger aggregates, and the studies proposed here will attempt to understand this process, its mechanism and its control at the molecular level. Based on the structural information from these studies, additional control and stability features can be engineered into the protein to expand and improve its biomedical applications.